1. Field of the Invention
The present invention relates generally to a dual-antenna system, and more particularly, the present invention relates to a leaky-wave dual-antenna system which can improve the mutual coupling S21, or isolation between antennas.
2. Description of the Prior Art
A conventional frequency-modulated continuous-wave (FM-CW) radar uses a single-antenna with a circulator or a dual-antenna structure to isolate the leakage power between transmitting and receiving ends. Furthermore, a leaky-wave type antenna with differential input could be used to further enhance the isolation effect. In the single antenna with a circulator design, the isolation between transmitting and receiving end is around −35 dB, and amplifiers can not be used between antenna and circulator. Also, the impedance mismatch between antenna and circulator will also result in more signal leakage. A dual-antenna structure has advantages of better isolation, however, will need more antenna area.
For example, a small leaky-wave antenna system 1 in FIG. 1 has only one transmitting and one receiving antenna element. The radiation field of a single element transmitting antenna 10 is illustrated in FIG. 2. As the beam width is too large, part of the energy radiated by the transmitting antenna 10 will be coupled directly to the nearby receiving antenna 12, and will degrade the receiver sensitivity. The mutual coupling S21 of the leaky-wave antenna system in FIG. 1 is simulated and shown in FIG. 3. The mutual coupling S21 could be defined as 20*log(V2/V1),  wherein V1 is an input voltage of the input end 100 of the transmitting antenna 10, V2 is an output voltage of the output end 120 of the receiving antenna 12. Generally, V2 is smaller than V1, so the coupling quantity S21 is a negative value. The maximum coupling factor represents the maximum energy, under the operating frequency band, which will be received by the receiving antenna via the coupling path of the transmitting antenna, and it is the smaller the better. As shown in FIG. 3, the coupling factor S21 under the operating frequency A1 (about 5.3 GHz˜5.4 GHz) of a conventional leaky-wave dual-antenna system 1 is greater than −30 dB, and the maximum coupling quantity is approximately −20 dB, which implies the leakage (coupling) between the transmitting antenna 10 and the receiving antenna 12 is too large.
Accordingly, the present invention provides a leaky-wave dual-antenna system which can reduce the maximum coupling factor under the operating frequency band to improve the leakage performance of an FMCW radar system.